The present invention relates to improvement in speed control systems for automobiles. More particularly, the invention relates to a speed control system which maintains the speed of automobiles at a stabilized desired value by employing a negative feedback loop.
Speed controls for automobiles are now in commercial use, in which the actual speed of an automobile is compared with a preselected desired value to generate an error signal for controlling a throttle valve to a position at which the actual speed corresponds to a preselected value. Generally speaking, automatic speed control of automobiles requires stabilization of the speed control system. If the stabilization should not be sufficient, so-called hunting, overshoot or undershoot of the actual speed of the automobile, occurs. The hunting can be prevented by deteriorating the responsibility and accuracy of the speed control system. However, the hunting can also be prevented without deteriorating this responsibility by employing negative feedback control of the throttle valve as shown in U.S. Pat. No. 3,381,771 (issued May 7, 1968). In the invention of the patent, an actual speed indication signal from a tachometer generator of an automobile speedometer and a preselected desired speed indication signal from a speed set potentiometer are supplied to a comparator circuit to generate an error signal to control the throttle valve position, then the error signal is supplied to a differential amplifier circuit together with a throttle position feedback signal (the negative feedback signal) from a feedback potentiometer linked with the throttle valve. An output signal from the differential amplifier circuit is supplied to a vacuum modulator which in response thereto provides to a vacuum motor vacuum pressure related to the output signal. The vacuum motor is mechanically ganged to the throttle valve and controls its position. Thus the output differential signal of the differential amplifier regulates the position of the throttle valve. The negative feedback of the throttle valve position prevents extra movement of the throttle valve toward opening or closing. Therefore, over-acceleration and -deceleration of the automobile are prevented. The desired speed set on the speed set potentiometer is altered by rotating a lever connected to the slider of the potentiometer. Similar speed controls for automobiles are disclosed in U.S. Pat. Nos. RE. 27,324 (issued Mar. 28, 1972), 3,477,346 (issued Nov. 11, 1969), 3,485,316 (issued Dec. 23, 1969), 4,056,157 (issued Nov. 1, 1977). A negative feedback circuit without use of a feedback potentiometer, and which compares a speed error signal with an actual automobile speed signal to generate a negative feedback signal, is disclosed in U.S. Pat. No. 3,952,829 (issued Apr. 27, 1976). Also a negative feedback circuit without use of a feedback potentiometer, and which has a circuit means for retarding the speed error signal to be supplied to a desired speed signal generator circuit as the negative feedback signal, is disclosed in U.S. Pat. No. 3,793,622 (issued Feb. 19, 1974). In the prior art, the systems for maintaining automobile speed constant have a speed memorization capacitor to which the first electric signal, indicating actual speed, is applied, and a set switch to close the charging loop of the capacitor for memorization of actual speed as seen in U.S. Pat. No. 3,485,316; and the memorized voltage level of the capacitor is supplied to the comparator as a desired speed indication signal. The desired speed set on the capacitor is altered by accelerating or decelerating the automobile and momentarily closing the set switch at the desired accelerated or decelerated speed. To accelerate speed automatically to alter the desired speed set on the capacitor, one or more switches, as well as one or more additional drive circuits to operate the throttle valve actuator toward opening of the throttle valve, are required. Thus, a plurality of set switches and drive circuits are required automatically to alter the desired speed set on the capacitor by switch operation.